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Aviation History
1918
1918 - 0567.PDF
MAY 23, 1918. .••: •,;; •;•' ',:-:::.:-:. •;•..,• ."::'; • -•' A NEW COMPASS DEVIATION OF the many and unfamiliar instruments to which the newly-hatched Flight Officer is introduced before becoming proficient in his profession, there is none which probably tends to puzzle him more at first than the compass when he is told that errors may exist on every point, and that corrections have to be applied before he can make good his course to steer. These errors are due to what is known as deviation of the compass, or the effect on it of the materials of which the aircraft is built, which varies in every direction to which it is pointed, and no compass can be considered worthy of any reliance until the machine to which it is fitted has been adjusted and " swung" to ascertain the remaining deviation, and the errors on each point ascertained and noted. Errors of 400 or 500 on some points of unadjusted aircraft compasses are not uncommon; hence the absolute necessity of all compasses being properly adjusted before • MACKENZIE • AERO-DEVIATION-CARD 7P1^ CARD FOR AIRCRAFT. are allowedmachinesto fly. The deviation of air- craft compasses is, as a rule, ascertained on the eight principal, points only, and the errors noted on a small card fixed near the compass for reference. Fig. 1 shows such a card in general use. STATION COMPASS For Magnetic Course, North N.E. East S.E. South S.W. West N.W. O 46° 9O° 136° 18O° 226° 27O° 315° Steer by C ompass. 3S8. I/O. 87- /So.132. 273.s/s. It occurred to Lieut. Sir Kenneth Mackenzie/ Bart.r R.N.V.R., attached to the R.N.A.S., as a Group Compass Officer, to overcome the difficulty by the simple expedient of marking a replica of the compass card with the deviations ascertained by " swinging " and calculation. The pilot would then have nothing to do but follow the markings on the card, and so avoid all chance of making any error in applying his deviation. The idea seemed so simple that he thougnt it must have been done long ago, but probably dis- carded for some reason or the other. This evidently was not the case, for when submitted to the Admiralty • Compass Observatory the device was at once adopted, and deviation cards on this principle have now beer*' issued for general use to the Air Services. Fig. 2 shows the new card with the same deviation* errors as are given in Fig. i, only extended throughout the whole 36 " points " in- stead of the eight principal ones only. In practice, such . a card might be markedjin — colour to make it noticeable, DEVIATION- 'TAB LE "- • Fig. 1.—A small card isusually employed, on which are noted the de-viations on the eight prin- cipal points. This card isgenerally fixed near the compass. •TOUSECARD AFTER • ASCERTAINING DEVIATION • DRAW- UNE5FR0W MAGNETIC COURSE TO MAKE-GOOD- TO -COMPASS- COURSE TO STEER.- Fig. 2.—The Mackenzie compass deviation card foraircraft. SHIPSHEAD N. N.E. E. S.E. 5. sw. w. N.W. OEVH Z £ S~ £ 3 £ / £ 7 *V 3 fiS / h^ Fig. 3.—The back of the Mackenzie-deviation card. Here the devia- tions on the eight points are notedby the adjuster for reference, in case the face of the card becomes,soiled. The errors on all the intermediate degrees, or each division of io° into which aircraft compass cards are now divided, would have to be calculated, or guessed, by a pilot whilst flying from the eight whose errors are given in order to make good any course on them he might have to steer. Even the experienced pilot has, in a modern aircraft, so much to do in the way of handling his machine and keeping an eye on the numerous " gadgets" in his cockpit, that he can only glance at his compass now and then either to see that he is going in the desired direction, or to take bearings of distant objects, as he has often to do. He is "fully occupied whilst doing this without having to calculate each time what the error may be on any particular point not included in the eight which are given in the old deviation cards, and apply it to find the course he should steer by compass. 565 but we have shown the markings in dotted lines for' facility of printing. Fig. 3 shows the back of the same card where the deviations on the eight points are noted by the adjuster for reference in case the face of the card, becomes soiled. These cards are being issued in two sizes; a 3-inch' compass circle for small aeroplanes where there is. not much room on the instrument board, and a 4-inch, for larger machines and airships. Sufficient has, we think, been said to show that the Mackenzie Aero Deviation Card should prove a boon to flying men who more and more will have to rely on their compasses as flights become longer and longer, especially when out of sight of land or any- thing to guide them. We may add that a deviation card on similar lines,
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